http://en.wikipedia.org/wiki/ChecksumA checksum is basically a mathmatical representation of the file prior to copying. It is a known value and checks it after copying to ensure it is valid.
Basically the same as a MD5 hash for a file on your computer (virus programs use these all the time).
For example - run.exe (made up file - is hashed and it has a known static value). If one part of the code is modified or even a space added, the hash will have changed and wont match
